Apparatus, systems, and methods for the fixation or fusion of bone

ABSTRACT

Assemblies of one or more implant structures make possible the achievement of diverse interventions involving the fusion and/or stabilization of the SI-joint and/or lumbar and sacral vertebra in a non-invasive manner, with minimal incision, and without the necessitating the removing the intervertebral disc. The representative lumbar spine interventions, which can be performed on adults or children, include, but are not limited to, SI-joint fusion or fixation; lumbar interbody fusion; translaminar lumbar fusion; lumbar facet fusion; trans-iliac lumbar fusion; and the stabilization of a spondylolisthesis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/952,102, filed Apr. 12, 2018, now abandoned, which is a continuationof U.S. patent application Ser. No. 15/195,955, filed Jun. 28, 2016, nowU.S. Pat. No. 9,949,843, titled “APPARATUS, SYSTEMS, AND METHODS FOR THEFIXATION OR FUSION OF BONE”, which is a continuation-in-part of U.S.patent application Ser. No. 13/858,814, filed Apr. 8, 2013, titled“APPARATUS, SYSTEMS, AND METHODS FOR ACHIEVING TRANS-ILIAC LUMBARFUSION,” now U.S. Pat. No. 9,375,323, which is a continuation of U.S.patent application Ser. No. 12/960,831, filed Dec. 6, 2010, titled“APPARATUS, SYSTEMS, AND METHODS FOR ACHIEVING TRANS-ILIAC LUMBARFUSION,” now U.S. Pat. No. 8,414,648, which is a continuation-in-part ofU.S. patent application Ser. No. 11/136,141, filed May 24, 2005, titled“SYSTEMS AND METHODS FOR THE FIXATION OR FUSION OF BONE,” now U.S. Pat.No. 7,922,765, which is a continuation-in-part of U.S. patentapplication Ser. No. 10/914,629, filed Aug. 9, 2004, titled “SYSTEMS ANDMETHODS FOR THE FIXATION OR FUSION OF BONE,” U.S. Patent Publication No.2006-003625-A1, now abandoned.

U.S. patent application Ser. No. 15/952,102, filed Apr. 12, 2018, is acontinuation of U.S. patent application Ser. No. 15/195,955, filed Jun.28, 2016, titled “APPARATUS, SYSTEMS, AND METHODS FOR THE FIXATION ORFUSION OF BONE”, now U.S. Pat. No. 9,949,843, which is also acontinuation-in-part of U.S. patent application Ser. No. 14/274,486,filed May 9, 2014, now U.S. Pat. No. 9,486,264, which is a continuationof U.S. patent application Ser. No. 13/786,037, filed Mar. 5, 2013,titled “SYSTEMS AND METHODS FOR THE FIXATION OR FUSION OF BONE USINGCOMPRESSIVE IMPLANTS,” now U.S. Pat. No. 8,734,462, which is acontinuation of U.S. patent application Ser. No. 12/924,784, filed Oct.5, 2010, titled “SYSTEMS AND METHODS FOR THE FIXATION OR FUSION OF BONEUSING COMPRESSIVE IMPLANTS,” now U.S. Pat. No. 8,388,667, which is acontinuation-in-part of U.S. patent application Ser. No. 11/136,141,filed May 24, 2005, titled “SYSTEMS AND METHODS FOR THE FIXATION ORFUSION OF BONE,” now U.S. Pat. No. 7,922,765 B2, each of which areherein incorporated by reference in their entirety.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

FIELD

This application relates generally to the fixation or fusion of bone.

BACKGROUND

Many types of hardware are available both for the fixation of bones thatare fractured and for the fixation of bones that are to fused(arthrodesed).

For example, the human hip girdle (see FIGS. 9 and 10 ) is made up ofthree large bones joined by three relatively immobile joints. One of thebones is called the sacrum and it lies at the bottom of the lumbarspine, where it connects with the L5 vertebra. The other two bones arecommonly called “hip bones” and are technically referred to as the rightilium and the left ilium. The sacrum connects with both hip bones at thesacroiliac joint (in shorthand, the SI-Joint).

The SI-Joint functions in the transmission of forces from the spine tothe lower extremities, and vice-versa. The SI-Joint has been describedas a pain generator for up to 22% of lower back pain.

To relieve pain generated from the SI Joint, sacroiliac joint fusion istypically indicated as surgical treatment, e.g., for degenerativesacroiliitis, inflammatory sacroiliitis, iatrogenic instability of thesacroiliac joint, osteitis condensans ilii, or traumatic fracturedislocation of the pelvis. Currently, screw and screw with plates areused for sacro-iliac fusion. At the same time the cartilage has to beremoved from the “synovial joint” portion of the SI joint. This requiresa large incision to approach the damaged, subluxed, dislocated,fractured, or degenerative joint.

The spine (see FIG. 37 ) is a complex interconnecting network of nerves,joints, muscles, tendons and ligaments, and all are capable of producingpain.

The spine is made up of small bones, called vertebrae. The vertebraeprotect and support the spinal cord. They also bear the majority of theweight put upon the spine.

Between each vertebra is a soft, gel-like “cushion,” called anintervertebral disc. These flat, round cushions act like shock absorbersby helping absorb pressure and keep the bones from rubbing against eachother. The intervertebral disc also binds adjacent vertebrae together.The intervertebral discs are a type of joint in the spine.Intervertebral disc joints can bend and rotate a bit but do not slide asdo most body joints.

Each vertebra has two other sets of joints, called facet joints (seeFIG. 38 ). The facet joints are located at the back of the spine(posterior). There is one facet joint on each lateral side (right andleft). One pair of facet joints faces upward (called the superiorarticular facet) and the other pair of facet joints faces downward(called the inferior articular facet). The inferior and superior facetjoints mate, allowing motion (articulation), and link vertebraetogether. Facet joints are positioned at each level to provide theneeded limits to motion, especially to rotation and to prevent forwardslipping (spondylolisthesis) of that vertebra over the one below.

In this way, the spine accommodates the rhythmic motions required byhumans to walk, run, swim, and perform other regular movements. Theintervertebral discs and facet joints stabilize the segments of thespine while preserving the flexibility needed to turn, look around, andget around.

Degenerative changes in the spine can adversely affect the ability ofeach spinal segment to bear weight, accommodate movement, and providesupport. When one segment deteriorates to the point of instability, itcan lead to localized pain and difficulties. Segmental instabilityallows too much movement between two vertebrae. The excess movement ofthe vertebrae can cause pinching or irritation of nerve roots. It canalso cause too much pressure on the facet joints, leading toinflammation. It can cause muscle spasms as the paraspinal muscles tryto stop the spinal segment from moving too much. The instabilityeventually results in faster degeneration in this area of the spine.

Degenerative changes in the spine can also lead to spondylolysis andspondylolisthesis. Spondylolisthesis is the term used to describe whenone vertebra slips forward on the one below it. This usually occursbecause there is a spondylolysis (defect) in the vertebra on top. Forexample, a fracture or a degenerative defect in the interarticular partsof lumbar vertebra L1 may cause a forward displacement of the lumbarvertebra L5 relative to the sacral vertebra S1 (called L5-S1spondylolisthesis). When a spondylolisthesis occurs, the facet joint canno longer hold the vertebra back. The intervertebral disc may slowlystretch under the increased stress and allow other upper vertebra toslide forward.

An untreated persistent, episodic, severely disabling back pain problemcan easily ruin the active life of a patient. In many instances, painmedication, splints, or other normally-indicated treatments can be usedto relieve intractable pain in a joint. However, in for severe andpersistent problems that cannot be managed by these treatment options,degenerative changes in the spine may require a bone fusion surgery tostop both the associated disc and facet joint problems.

A fusion is an operation where two bones, usually separated by a joint,are allowed to grow together into one bone. The medical term for thistype of fusion procedure is arthrodesis.

Lumbar fusion procedures have been used in the treatment of pain and theeffects of degenerative changes in the lower back. A lumbar fusion is afusion in the S1-L5-L4 region in the spine.

One conventional way of achieving a lumbar fusion is a procedure calledanterior lumbar interbody fusion (ALIF). In this procedure, the surgeonworks on the spine from the front (anterior) and removes a spinal discin the lower (lumbar) spine. The surgeon inserts a bone graft into thespace between the two vertebrae where the disc was removed (theinterbody space). The goal of the procedure is to stimulate thevertebrae to grow together into one solid bone (known as fusion). Fusioncreates a rigid and immovable column of bone in the problem section ofthe spine. This type of procedure is used to try and reduce back painand other symptoms.

Facet joint fixation procedures have also been used for the treatment ofpain and the effects of degenerative changes in the lower back. Theseprocedures take into account that the facet joint is the only truearticulation in the lumbosacral spine. In one conventional procedure forachieving facet joint fixation, the surgeon works on the spine from theback (posterior). The surgeon passes screws from the spinous processthrough the lamina and across the mid-point of one or more facet joints.

Conventional treatment of spondylolisthesis may include a laminectomy toprovide decompression and create more room for the exiting nerve roots.This can be combined with fusion using, e.g., an autologous fibulargraft, which may be performed either with or without fixation screws tohold the bone together. In some cases the vertebrae are moved back tothe normal position prior to performing the fusion, and in others thevertebrae are fused where they are after the slip, due to the increasedrisk of injury to the nerve with moving the vertebra back to the normalposition.

Currently, these procedures entail invasive open surgical techniques(anterior and/or posterior). Further, ALIF entails the surgical removalof the disc. Like all invasive open surgical procedures, such operationson the spine risk infections and require hospitalization. Invasive opensurgical techniques involving the spine continue to be a challenging anddifficult area.

SUMMARY OF THE DISCLOSURE

Embodiments of the invention provide bone fixation/fusion systems,devices, and related methods for stabilizing adjacent bone segments in aminimally invasive manner. The adjacent bone segments can comprise partsof the same bone that have been fractured, or two or more individualbones separated by a space or joint. As used herein, “bone segments” or“adjacent bone regions” refer to either situation, i.e., a fracture linein a single bone (which the devices serve to fixate), or a space orjoint between different bone segments (which the devices serve toarthrodese or fuse). The devices can therefore serve to perform afixation function between two or more individual bones, or a fusionfunction between two or more parts of the same bone, or both functions.

One aspect of the invention provides assemblies and associated methodsfor the fixation or fusion of bone structures comprising first andsecond bone segments separated by a fracture line or joint. Theassemblies and associated methods comprise an anchor body sized andconfigured to be introduced into the first and second bone segments. Theanchor body has a distal end located in an interior region of the secondbone segment; a proximal end located outside an exterior region of thefirst bone segment; and an intermediate region spanning the fractureline or joint between the first and second bone segments. The assembliesand associated methods also include a distal anchor secured to theinterior region of the second bone segment and affixed to the distal endof the anchor body to anchor the distal end in the second bone segment.The assemblies and associated methods further include a proximal anchorsecured to the exterior region of the first bone segment and affixed tothe proximal end of the anchor body, which, in concert with the distalanchor, places the anchor body in compression to compress and fixate thebone segments relative to the fracture line or joint. The assemblies andassociated methods also include an elongated implant structure carriedby the intermediate region of the anchor body and spanning the fractureline or joint between the bone segments. The elongated implant structureincludes an exterior surface region treated to provide bony in-growth orthrough-growth along the implant structure, to accelerate the fixationor fusion of the first and second bone segments held in compression andfixated by the anchor body.

The bone fixation/fusion systems, devices, and related methods are wellsuited for stabilizing adjacent bone segments in the SI-Joint.

Accordingly, another aspect of the invention provides a method for thefusion of the sacral-iliac joint between an iliac and a sacrum. Themethod comprises creating an insertion path through the ilium, throughthe sacral-iliac joint, and into the sacrum. The method includesproviding an anchor body sized and configured to be introduced throughthe insertion path laterally into the ilium and sacrum. The anchor bodyhas a distal end sized and configured to be located in an interiorregion of the sacrum; a proximal end sized and configured to be locatedoutside an exterior region of the iliac; and an intermediate regionsized and configured to span the sacral-iliac joint. The method includesproviding an elongated implant structure sized and configured to bepassed over the anchor body to span the sacral-iliac joint between theiliac and sacrum. The elongated implant structure includes an exteriorsurface region treated to provide bony in-growth or through-growth alongthe implant structure. The method includes introducing the anchor bodythrough the insertion path from the ilium, through the sacral-iliacjoint, and into the sacrum. The method includes anchoring the distal endof the anchor body in the interior region of the sacrum. The methodincludes passing the elongated implant structure over the anchor body tospan the sacral-iliac joint between the ilium and sacrum, and anchoringthe proximal end of the anchor body to an exterior region of the ilium,which, in concert with the anchored distal end, places the anchor bodyin compression to compress and fixate the sacral-iliac joint. The bonyin-growth or through-growth region of the implant structure acceleratesthe fixation or fusion of the sacral-iliac joint held in compression andfixated by the anchor body.

Embodiments of the invention provide apparatus, systems, and methods forthe fusion and/or stabilization of the lumbar spine. The apparatus,systems, and methods include one or more elongated, stem-like implantstructures sized and configured for the fusion or stabilization ofadjacent bone structures in the lumbar region of the spine, eitheracross the intervertebral disc or across one or more facet joints. Eachimplant structure includes a region formed along at least a portion ofits length to promote bony in-growth onto or into surface of thestructure and/or bony growth entirely through all or a portion of thestructure. The bony in-growth or through-growth region along the surfaceof the implant structure accelerates bony in-growth or through-growthonto, into, or through the implant structure 20. The implant structuretherefore provides extra-articular/intra osseous fixation, when bonegrows in and around the bony in-growth or through-growth region. Bonyin-growth or through-growth onto, into, or through the implant structurehelps speed up the fusion and/or stabilization process of the adjacentbone regions fixated by the implant structure.

The assemblies of one or more implant structures make possible theachievement of diverse interventions involving the fusion and/orstabilization of lumbar and sacral vertebra in a non-invasive manner,with minimal incision, and without the necessitating the removing theintervertebral disc. The representative lumbar spine interventions,which can be performed on adults or children, include, but are notlimited to, lumbar interbody fusion; translaminar lumbar fusion; lumbarfacet fusion; trans-iliac lumbar fusion; and the stabilization of aspondylolisthesis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side section view of a compression stem assembly assembledin adjacent bone regions, which are shown in FIG. 1 in adiagrammatically fashion for the purpose of illustration, withoutanatomic detail, which is later shown, e.g., in FIG. 16 .

FIG. 2 is an exploded perspective view of the components of thecompression stem assembly shown in FIG. 1 prior to assembly.

FIGS. 3 to 7 are alternative embodiments of an implant structure whichforms a part of the compression stem assembly shown in FIGS. 1 and 2 ,illustrating different cross-sectional geometries and configurations forthe implant structure 20.

FIGS. 8A to 8L are side section views of the introduction and assemblyof the compression stem assembly shown in FIGS. 1 and 2 , which is shownin FIGS. 8A to 8L in a diagrammatically fashion for the purpose ofillustration, without anatomic detail, as later shown, e.g., in FIG. 16.

FIGS. 9 and 10 are, respectively, anterior and posterior anatomic viewsof the human hip girdle comprising the sacrum and the hip bones (theright ilium, and the left ilium), the sacrum being connected with bothhip bones at the sacroiliac joint (in shorthand, the SI-Joint).

FIGS. 11 to 13A and 13B are anatomic views showing, respectively, inexploded perspective, assembled perspective, assembled anterior view,and assembled axial section view, the implantation of three implantstructures, without association of a compression stem assembly, for thefixation of the SI-Joint using a lateral approach laterally through theilium, the SI-Joint, and into the sacrum S1.

FIGS. 14 to 16A and 16B are anatomic views showing, respectively, inexploded perspective, assembled perspective, assembled anterior view,and assembled axial section view, the implantation of three implantstructures, in association with a compression stem assembly, for thefixation of the SI-Joint using a lateral approach laterally through theilium, the SI-Joint, and into the sacrum S1.

FIGS. 17 to 19A and 19B are anatomic views showing, respectively, inexploded perspective, assembled perspective, assembled lateral view, andassembled axial section view, the implantation of three implantstructures, without association of a compression stem assembly, for thefixation of the SI-Joint using a postero-lateral approach entering fromthe posterior iliac spine of the ilium, angling through the SI-Joint,and terminating in the sacral alae.

FIGS. 20 to 22A and 22B are anatomic views showing, respectively, inexploded perspective, assembled perspective, assembled lateral view, andassembled axial section view, the implantation of three implantstructures, in association with a compression stem assembly, for thefixation of the SI-Joint using a postero-lateral approach entering fromthe posterior iliac spine of the ilium, angling through the SI-Joint,and terminating in the sacral alae.

FIGS. 23 and 24A and 24B are anatomic views showing, respectively, inexploded perspective, assembled anterior view, and assembled axialsection view, the implantation of a screw-like structure for thefixation of the SI-Joint using a lateral approach laterally through theilium, the SI-Joint, and into the sacrum S1.

FIGS. 25 and 26A and 26B are anatomic views showing, respectively, inexploded perspective, assembled lateral view, and assembled axialsection view, the implantation of a screw-like structure for thefixation of the SI-Joint using a postero-lateral approach entering fromthe posterior iliac spine of the ilium, angling through the SI-Joint,and terminating in the sacral alae.

FIGS. 27 and 28A and 28B are anatomic views showing, respectively, inexploded perspective, assembled anterior view, and assembled axialsection view, the implantation of a fusion cage structure for thefixation of the SI-Joint using a lateral approach laterally through theilium, the SI-Joint, and into the sacrum S1.

FIGS. 29 and 30A and 30B are anatomic views showing, respectively, inexploded perspective, assembled lateral view, and assembled axialsection view, the implantation of a fusion cage structure for thefixation of the SI-Joint using a postero-lateral approach entering fromthe posterior iliac spine of the ilium, angling through the SI-Joint,and terminating in the sacral alae.

FIG. 31 is an exploded perspective view of the components of analternative embodiment of a compression stem assembly prior to assembly.

FIGS. 32 and 33 are perspective views of the alternative embodiment of acompression stem assembly shown in FIG. 31 after assembly, showingrotation of an anchor plate associated with the assembly from an alignedposition (FIG. 32 ) to a bone-gripping position (shown in FIG. 33 ), toanchor the assembly in bone.

FIG. 34 is a side section view of the compression stem assembly shown inFIG. 31 assembled in adjacent bone regions, which are shown in FIG. 34in a diagrammatically fashion for the purpose of illustration, withoutanatomic detail.

FIGS. 35A and 35B are side section views of an alternative embodiment ofa compression stem assembly prior to assembly (FIG. 35A) and afterassembly (FIG. 35B) in adjacent bone regions, which are shown in FIGS.35A and 35B in a diagrammatically fashion for the purpose ofillustration, without anatomic detail.

FIGS. 36A and 36B are side section views of a radially compressibleimplant prior to assembly (FIG. 36A) and after assembly (FIG. 36B) inadjacent bone regions, which are shown in FIGS. 36A and 36B in adiagrammatically fashion for the purpose of illustration, withoutanatomic detail.

FIG. 37 is an anatomic anterior and lateral view of a human spine.

FIG. 38 is an anatomic posterior perspective view of the lumbar regionof a human spine, showing lumbar vertebrae L2 to L5 and the sacralvertebrae.

FIG. 39 is an anatomic anterior perspective view of the lumbar region ofa human spine, showing lumbar vertebrae L2 to L5 and the sacralvertebrae.

FIG. 40 is a perspective view of a representative embodiment of anelongated, stem-like, cannulated implant structure well suited for thefusion or stabilization of adjacent bone structures in the lumbar regionof the spine, either across the intervertebral disc or across one ormore facet joints.

FIGS. 41 to 44 are perspective views of other representative embodimentsof implant structures well suited for the fusion or stabilization ofadjacent bone structures in the lumbar region of the spine, eitheracross the intervertebral disc or across one or more facet joints.

FIG. 45 is an anatomic anterior perspective view showing, in an explodedview prior to implantation, a representative configuration of anassembly of one or more implant structures as shown in FIG. 40 , sizedand configured to achieve anterior lumbar interbody fusion, in anon-invasive manner and without removal of the intervertebral disc.

FIG. 46 is an anatomic anterior perspective view showing the assemblyshown in FIG. 45 after implantation.

FIG. 47 is an anatomic right lateral perspective view showing theassembly shown in FIG. 45 after implantation.

FIG. 48 is an anatomic superior left lateral perspective view showingthe assembly shown in FIG. 45 after implantation.

FIGS. 49A to 49G are diagrammatic views showing, for purposes ofillustration, a representative lateral (or posterolateral) procedure forimplanting the assembly of implant structures shown in FIGS. 46 to 48 .

FIG. 50 is an anatomic anterior perspective view showing, in an explodedview prior to implantation, assemblies comprising one or more implantstructures like that shown in FIG. 40 inserted from left and/or rightanterolateral regions of a given lumbar vertebra, in an angled paththrough the intervertebral disc and into an opposite anterolateralinterior region of the next inferior lumbar vertebra, FIG. 50 showing inparticular two implant structures entering on the right anterolateralside of L4, through the intervertebral disc and into the leftanterolateral region of L5, and one implant structure entering on theleft anterolateral side of L4, through the intervertebral disc and intothe right anterolateral region of L5, the left and right implantstructures crossing each other in transit through the intervertebraldisc.

FIG. 51 is an anatomic anterior perspective view showing, in an explodedview prior to implantation, assemblies comprising one or more implantstructures like that shown in FIG. 40 inserted from left and/or rightanterolateral regions of a given lumbar vertebra, in an angled paththrough the intervertebral disc and into an opposite anterolateralinterior region of the next inferior lumbar vertebra, FIG. 50 showing inparticular one implant structure entering on the right anterolateralside of L4, through the intervertebral disc and into the leftanterolateral region of L5, and one implant structure entering on theleft anterolateral side of L4, through the intervertebral disc and intothe right anterolateral region of L5, the left and right implantstructures crossing each other in transit through the intervertebraldisc.

FIG. 52 is an anatomic posterior perspective view, exploded prior toimplantation, of a representative configuration of an assembly of one ormore implant structures like that shown in FIG. 40 , sized andconfigured to achieve translaminar lumbar fusion in a non-invasivemanner and without removal of the intervertebral disc.

FIG. 53 is an anatomic inferior transverse plane view showing theassembly shown in FIG. 52 after implantation.

FIG. 54 is an anatomic posterior perspective view, exploded prior toimplantation, of a representative configuration of an assembly of one ormore implant structures like that shown in FIG. 40 , sized andconfigured to achieve lumbar facet fusion, in a non-invasive manner andwithout removal of the intervertebral disc.

FIG. 55 is an anatomic inferior transverse plane view showing theassembly shown in FIG. 54 after implantation.

FIG. 56 is an anatomic lateral view showing the assembly shown in FIG.54 after implantation.

FIG. 57A is an anatomic anterior perspective view showing, in anexploded view prior to implantation, a representative configuration ofan assembly of one or more implant structures like that shown in FIG. 40, sized and configured to achieve fusion between lumbar vertebra L5 andsacral vertebra S1, in a non-invasive manner and without removal of theintervertebral disc, using an anterior approach.

FIG. 57B is an anatomic anterior perspective view showing the assemblyshown in FIG. 57A after implantation.

FIG. 58A is an anatomic posterior view showing, in an exploded viewprior to implantation, another representative configuration of anassembly of one or more implant structures sized and configured toachieve fusion between lumbar vertebra L5 and sacral vertebra S1, in anon-invasive manner and without removal of the intervertebral disc,using a postero-lateral approach entering from the posterior iliac spineof the ilium, angling through the SI-Joint, and terminating in thelumbar vertebra L5.

FIG. 58B is an anatomic posterior view showing the assembly shown inFIG. 58A after implantation.

FIG. 58C is an anatomic superior view showing the assembly shown in FIG.58B.

FIG. 59 is an anatomic lateral view showing a spondylolisthesis at theL5/S1 articulation, in which the lumbar vertebra L5 is displaced forward(anterior) of the sacral vertebra S1.

FIG. 60A is an anatomic anterior perspective view showing, in anexploded view prior to implantation, a representative configuration ofan assembly of one or more implant structures like that shown in FIG. 40, sized and configured to stabilize a spondylolisthesis at the L5/S1articulation.

FIG. 60B is an anatomic anterior perspective view showing the assemblyshown in FIG. 60A after implantation.

FIG. 60C is an anatomic lateral view showing the assembly shown in FIG.60B.

DETAILED DESCRIPTION

Although the disclosure hereof is detailed and exact to enable thoseskilled in the art to practice the invention, the physical embodimentsherein disclosed merely exemplify the invention that may be embodied inother specific structure. While the preferred embodiment has beendescribed, the details may be changed without departing from theinvention, which is defined by the claims.

Part I

The following describes embodiments of the invention for use in thefixation or fusion of the SI-joint and other bone segments or joints.

I. The Compression Stem Assembly

FIGS. 1 and 2 show in assembled and exploded views, respectively, arepresentative configuration of a compression stem assembly 10 sized andconfigured for the fixation of bone fractures (i.e., fixation of partsof the same bone) or for the fixation of bones which are to be fused(arthrodesed) (i.e. fixation of two or more individual bones that areadjacent and/or jointed). For the sake of shorthand, the assembly 10will sometimes be called a bone fixation/fusion compression assembly, toindicate that it can perform a fixation function between two or moreindividual bones), or a fusion function between two or more parts of thesame bone, or both functions. As used herein, “bone segments” or“adjacent bone regions” refer to either situation, i.e., a fracture linein a single bone or a space or joint between different bone segments. InFIG. 1 , the bone segment or adjacent bone regions are showndiagrammatically without anatomic detail for the purpose ofillustration. Later, e.g., in FIGS. 13 to 16 and FIGS. 20 to 22 , thebone segments or adjacent bone regions are shown in a specific anatomicsetting, comprising the joint between the sacrum and the ilium of thepelvis, also anatomically called the sacroiliac joint (SI-Joint).

As shown in FIGS. 1 and 2 , the compression stem assembly 10 comprisesan anchor body 12, which (as shown in FIG. 1 ) is sized and configuredto be placed in compression within bone segments or adjacent boneregions. In a representative embodiment, the anchor body 12 takes theform of a cylindrical anchor pin or rod. Still, the anchor body 12 canpossess other geometries.

The anchor body 12 is anchored at a distal end to a distal anchor screw14 coupled to an interior bone region in one side of the space or joint.The anchor body 12 is secured at a proximal end, on the opposite side ofthe space or joint, to an exterior bone region by an anchor nut 16 andanchor washer 18. The distal anchor screw 14 and anchor nut 16 hold theanchor body 12 in compression and, in doing so, the anchor body 12compresses and fixates the bone segments or adjacent bone regions.

The anchor body 12 carries within the bone regions or segments anelongated, stem-like, cannulated implant structure 20. The implantstructure 20 includes an interior bore 22 that accommodates itsplacement by sliding over the anchor body 12. As FIG. 2 shows, theimplant structure 20 includes a region 24 formed along at least aportion of its length to promote bony in-growth onto or into surface ofthe structure and/or bony growth entirely through all or a portion ofthe structure. The bony-in-growth or through-growth region 24 along thesurface of the implant structure 20 accelerates bony in-growth orthrough-growth onto, into, or through the implant structure 20. Bonyin-growth or through-growth onto, into, or through the implant structure20 helps speed up the fusion process or fracture healing time of thebone segments or adjacent bone regions held in compression and fixatedby the anchor body 12.

A. The Anchor Body, Nut, and Washer

The anchor body 12, nut 16, and washer 18 can be formed—e.g., bymachining, molding, or extrusion—from a material usable in theprosthetic arts that is capable of being placed into and holdingcompressive forces and that is not subject to significant bio-absorptionor resorption by surrounding bone or tissue over time. The anchor body12, nut 16, and washer 18 are intended to remain in place for a timesufficient to stabilize the fracture or fusion site. Examples of suchmaterials include, but are not limited to, titanium, titanium alloys,tantalum, chrome cobalt, surgical steel, or any other total jointreplacement metal and/or ceramic, sintered glass, artificial bone, anyuncemented metal or ceramic surface, or a combination thereof.

In length (see FIG. 1 ), the anchor body 12 is sized to span a distancethrough one adjacent bone segment or region, through the interveningspace or joint, and at least partially into the other adjacent bonesegment or region. The anchor body 12 is sized on length and diameteraccording to the local anatomy. The morphology of the local structurescan be generally understood by medical professionals using textbooks ofhuman skeletal anatomy along with their knowledge of the site and itsdisease or injury. The physician is also able to ascertain thedimensions of the anchor body 12 based upon prior analysis of themorphology of the targeted bone region using, for example, plain filmx-ray, fluoroscopic x-ray, or MRI or CT scanning. A representativediameter for the anchor body 12 can range between 3.2 mm to 3.5 mm.

As best shown in FIG. 2 , at least the proximal and distal regions ofthe anchor body 12 include external helical ridges or screw threads 26and 28 formed around the cylindrical body of the anchor body 12.Alternatively, the anchor body 12, if desired, can be threadedsubstantially along its entire length. Desirably, the direction of thescrew threads 26 and 28 is the same at both proximal and distal regionsof the anchor body 12, e.g., they desirably comprise right-hand threads.

The proximal region of the anchor body 12 carrying the threads 26 issized to extend, in use, a distance outside the one adjacent bonesegment or region. In this way, the proximal region is, in use, exposedso that the proximal anchor nut 16 and washer 18 can be attached. Theanchor nut 16 includes complementary internal screw threads that aresized and configured to mate with the external screw threads 26 on theproximal region of the anchor body 12. Representative diameters for ananchor nut 16 and anchor washer 18 for a 3.2 mm anchor body 12 are,respectively, 3.2 mm and 8 mm.

The distal region of the anchor body 12 carrying the threads 28 is sizedto extend at least partially into the other adjacent bone segment orregion, where it is to be coupled to the anchor screw 14, as will nextbe described.

B. The Anchor Screw

Like the anchor body 12, nut and washer 18, the anchor screw 14 canlikewise be formed—e.g., by machining, or molding—from a durablematerial usable in the prosthetic arts that is capable of being screwedinto bone and that is not subject to significant bio-absorption orresorption by surrounding bone or tissue over time. The anchor screw 14,like the other components of the compression assembly 10, is intended toremain in place for a time sufficient to stabilize the fracture orfusion site. Examples of such materials include, but are not limited to,titanium, titanium alloys, tantalum, chrome cobalt, surgical steel, orany other total joint replacement metal and/or ceramic, or a combinationthereof.

The anchor screw 14 is sized to span a distance within the otheradjacent bone segment or region at the terminus of the threaded distalregion 28 of the anchor body 12. As best shown in FIG. 2 , the anchorscrew 14 includes external helical ridges or screw threads 30 formedaround the cylindrical body of the anchor screw 14. The external screwthreads 30 are sized and configured to gain purchase in bone whenrotated, so that the anchor screw 14 can be advanced and seated byrotation into bone in the bone segment or region. The anchor screw 14,seated within the bone, resists axial migration and separation. Arepresentative range of lengths for the anchor screw 14 can be between 5mm to 20 mm, again depending upon the demands of the local anatomy. Arepresentative diameter for the anchor screw 14 is about 7 mm.

The anchor screw 14 also includes internal helical ridges or screwthreads 32 formed within a bore in the anchor screw 14. The internalscrew threads 32 are sized and configured to mate with the complementaryexternal screw threads 28 on the distal region of the anchor body 12.When threaded and mated to the internal screw threads 32 of the anchorscrew 14, the anchor screw 14 anchors the distal region of the anchorbody 12 to bone to resists axial migration of the anchor body 12. Asbefore described, the anchor screw 14 (on the distal end) and the anchornut 16 and anchor washer 18 (on the proximal end) hold the anchor body12 in compression, thereby compressing and fixating the bone segments oradjacent bone regions.

Alternatively, in place of the anchor screw 14, an internally threadedcomponent free external screw threads can be is sized and configured tobe securely affixed within the broached bore in the most distal bonesegment where the broached bore terminates, e.g., by making aninterference fit and/or otherwise being secured by the use of adhesives.Like the anchor screw 14, the interference fit and/or adhesives anchorthe overall implant structure. Adhesives may also be used in combinationwith the anchor screw 14.

C. The Implant Structure

The implant structure 20 can be formed—e.g., by machining, molding, orextrusion—from a durable material usable in the prosthetic arts that isnot subject to significant bio-absorption or resorption by surroundingbone or tissue over time. The implant structure 20, like the othercomponents of the compression assembly 10, is intended to remain inplace for a time sufficient to stabilize the fracture or fusion site.Such materials include, but are not limited to, titanium, titaniumalloys, tantalum, tivanium (aluminum, vanadium, and titanium), chromecobalt, surgical steel, or any other total joint replacement metaland/or ceramic, sintered glass, artificial bone, any uncemented metal orceramic surface, or a combination thereof. Alternatively, the implantstructure 20 may be formed from a suitable durable biologic material ora combination of metal and biologic material, such as a biocompatiblebone-filling material. The implant structure 20 may be molded from aflowable biologic material, e.g., acrylic bone cement, that is cured,e.g., by UV light, to a non-flowable or solid material.

The implant structure 20 is sized according to the local anatomy. Themorphology of the local structures can be generally understood bymedical professionals using textbooks of human skeletal anatomy alongwith their knowledge of the site and its disease or injury. Thephysician is also able to ascertain the dimensions of the implantstructure 20 based upon prior analysis of the morphology of the targetedbone region using, for example, plain film x-ray, fluoroscopic x-ray, orMRI or CT scanning.

As FIGS. 3 to 7 show, the implant structure 20 can take various shapesand have various cross-sectional geometries. The implant structure 20can have, e.g., a generally curvilinear (i.e., round or oval)cross-section—as FIG. 3 shows for purposes of illustration—or agenerally rectilinear cross section (i.e., square or rectangular ortriangular—as FIG. 4 shows for purposes of illustration—or combinationsthereof. In FIG. 2 , the implant structure 20 is shown to be triangularin cross section, which effectively resists rotation and micromotiononce implanted.

As FIGS. 5 and 6 show, the implant structure 20, whether curvilinear(FIG. 5 ) or rectilinear (FIG. 6 ) can include a tapered region 34 atleast along a portion of its axial length, meaning that the width ordiameter of the implant structure 20 incrementally increases along itsaxial length. Desirably, the tapered region 34 corresponds with, in use,the proximal region of the implant structure 20 (i.e., the last part ofthe implant structure 20 to enter bone). The amount of the incrementalincrease in width or diameter can vary. As an example, for an implantstructure 20 having a normal diameter of 7 mm, the magnitude of theincremental increase at its maximum can range between about 0.25 mm to1.25 mm. The tapered region 34 further enhances the creation andmaintenance of compression between the bone segments or regions.

To further enhance the creation and maintenance of compression betweenthe bone segments or regions (see FIG. 7 ), the implant structure 20,whether curvilinear or rectilinear or tapered, can include projectingbone-gripping surfaces 36 in the form of “teeth” or wings or the like.The teeth or wings 36 can project, e.g., 2 to 4 mm from the surface ofthe implant structure 20 and face in the direction of the compressionforces at proximal and distal ends of the implant structure 20, takingpurchase into the bone segments as they are compressed together by thecompression assembly.

The bony in-growth or through-growth region 24 may extend along theentire outer surface of the implant structure 20, as shown in FIG. 1 or2 , or the bony in-growth or through-growth region 24 may cover just aspecified distance on either side of the bone segments or fracture line.The bony in-growth region 24 or through-growth can comprise, e.g.,through holes, and/or various surface patterns, and/or various surfacetextures, and/or pores, or combinations thereof. The configuration ofthe bony in-growth or through-growth region 24 can, of course, vary. Byway of examples, the bony in-growth or through-growth region 24 cancomprise an open mesh configuration; or beaded configuration; or atrabecular configuration; or include holes or fenestrations. Anyconfiguration conducive to bony in-growth and/or bony through-growthwill suffice.

The bony in-growth or through-growth region 24 can be coated or wrappedor surfaced treated to provide the bony in-growth or through-growthregion, or it can be formed from a material that itself inherentlypossesses a structure conducive to bony in-growth or through-growth,such as a porous mesh, hydroxyapetite, or other porous surface. The bonyin-growth or through-growth region can include holes that allow bone togrow throughout the region.

In a preferred embodiment, the bony in-growth region or through-growthregion 24 comprises a porous plasma spray coating on the implantstructure 20. This creates a biomechanically rigorous fixation/fusionsystem, designed to support reliable fixation/fusion and acute weightbearing capacity.

The bony in-growth or through-growth region 24 may further be coveredwith various other coatings such as antimicrobial, antithrombotic, andosteoinductive agents, or a combination thereof. The entire implantstructure 20 may be impregnated with such agents, if desired.

D. Implantation of the Compression Stem Assembly

FIGS. 8A to 8L diagrammatically, show for purposes of illustration, arepresentative procedure for implanting a compression stem assembly 10.More detailed, anatomically-focused descriptions of particularimplantation techniques of the compression stem assembly 10 in theSI-Joint will be described later.

The physician identifies the bone segments or adjacent bone regions thatare to be fixated or fused (arthrodesed) (see FIG. 8A). Aided byconventional visualization techniques, e.g., using X-ray imageintensifiers such as a C-arms or fluoroscopes to produce a live imagefeed which is displayed on a TV screen, a guide pin 38 is introduced byconventional means (see FIG. 8B) through the one adjacent bone segmentor region, through the intervening space or joint, and partially intothe other adjacent bone segment or region.

A cannulated drill bit 40 is passed over the guide pin 38 (see FIG. 8C),to form a pilot insertion path or bore 42 through the one adjacent bonesegment or region, through the intervening space or joint, and partiallyinto the other adjacent bone segment or region. A single drill bit ormultiple drill bits 40 can be employed to drill through bone fragmentsor bone surfaces to create a pilot bore 42 of the desired size andconfiguration. A region of bone distal to the pilot bore 42 is leftundrilled and native for seating of the anchor screw 14. When the pilotbore 42 is completed, the cannulated drill bit 40 is removed.

A broach 44 having the external geometry and dimensions matching theexternal geometry and dimensions of the implant structure 20 (which, inthe illustrated embodiment, is triangular) (see FIG. 8D) is tapped overthe guide pin 38 through the pilot bore 42. The shaped broach 44 cutsalong the edges of the pilot bore 42 to form the desired profile (which,in the illustrated embodiment, is triangular) to accommodate the implantstructure 20 through the one adjacent bone segment or region, throughthe intervening space or joint, and partially into the other adjacentbone segment or region.

The broach 44 is withdrawn (see FIG. 8E), and the anchor screw 14 (itsinternal screw threads 32 mated to the distal end of a cannulatedthreaded screw driver 46) is passed over the guide pin 38 to theterminus of the broached bore 48 in the distal bone segment. The anchorscrew 14 is threaded by operation of the screw driver 46 (see FIG. 8F)into the undrilled and native bone beyond the terminus of the broachedbore 48. For example, the anchor screw 14 can be advanced and buried inbone at least 5 mm beyond the terminus of the broached bore 48.

The threaded screw driver 46 is unthreaded by reverse rotation from theanchor screw 14, and the guide pin 38 is removed (see FIG. 8G). Theanchor body 12 is inserted, and its threaded distal end 28 is threadedinto and mated with the internal screw threads 32 of the anchor screw 14(see FIG. 8H).

As shown in FIG. 8H, due to its purposeful size and configuration, whenits threaded distal end 28 is suitably threaded to the anchor screw 14,the threaded proximal end 26 of the anchor body 12 projects an exposeddistance outside the proximal end of the broached bore 48.

The implant structure 20 is passed over the anchor body 12 by sliding itover the anchor body 12. As FIG. 8I shows, the length of the implantstructure 20 selected is less than the distance between the anchor screw14 and the threaded proximal end 26, such that, when initially insertedand before compression is applied to the anchor body 26, the distal endof the implant structure 20 is spaced from the proximal end of theanchor screw 14 (see FIG. 8I). The distance can range, e.g., betweenabout 4 mm to about 10 mm.

The anchor washer 18 is passed by sliding over the exposed threadedproximal end 26 of the anchor body 12 into abutment against an exteriorbone surface (see FIG. 8J). The anchor nut 16 is threaded onto and matedto the threaded proximal end 26 of the anchor body 12 (see FIG. 8K). Theanchor nut 16 is tightened against the anchor washer 18 using a hand (orpowered) chuck 50 (see FIG. 8L), until a desired amount of compressionis applied to the bone regions by the assembly 10. The compression willreduce the distance between the bone segments (as FIGS. 8K and 8L show),as the distal end 28 of the anchor body 12, affixed to the anchor screw14 in the more distal bone segment, draws the more distal bone segmenttoward the more proximal bone segment, while eventually placing theimplant structure 20 itself into compression within the broached bore 48as the implant structure 20 comes into abutment against both the anchorwasher 18 and the anchor screw 14, assuring intimate contact between thebony in-growth region 24 and bone within the broached bore 48.

The intimate contact created by the compression between the bonyin-growth or through-growth region 24 along the surface of the implantstructure 20 accelerates bony in-growth or through-growth onto, into, orthrough the implant structure 20, to accelerate the fusion process orfracture healing time.

As will be described in greater detail later, more than one compressionstem assembly 10 can be implanted in a given bone segment. For example,as will be described later (see, e.g., FIG. 20 ), three such compressionstem assemblies can be implanted to fuse a SI-Joint.

E. Alternative Embodiments

1. Distal Anchor Plate

An alternative embodiment for the compression stem assembly 10 is shownin FIGS. 31 to 33 . In use, the compression stem assembly 10 is sizedand configured to be implanted in adjoining bone segments, which areseparated by a space or joint, for the purpose of bone fixation or jointfusion, as already described.

In this embodiment (see FIG. 31 ), the anchor body 12, nut 16, andwasher 18 are sized and configured as previously described. Likewise,the implant structure 20 is sized and configured with a generallyrectilinear cross section, as also earlier described and shown in FIG. 4.

In this embodiment, instead of a threaded anchor screw 14, the distalend of the assembly 10 is anchored into bone by a generally rectilinearanchor plate 58. The anchor plate 58 is formed—e.g., by machining, ormolding—from a hard, durable material usable in the prosthetic arts thatis capable of cutting into and gaining purchase in bone, and that is notsubject to significant bio-absorption or resorption by surrounding boneor tissue over time.

As best shown in FIGS. 31 and 32 , the rectilinear anchor plate 58 issized and configured to match the rectilinear cross section of theimplant structure itself. In the illustrated arrangement, the implantstructure 20 is generally triangular in cross section, and so, too, isthe anchor plate 58. As such, the anchor plate 58 includes apexes 64.The sides of the anchor plate 58 between the apexes are sharpened tocomprise bone cutting edges 72.

The anchor plate 58 also includes a bore 60 in its geometric center (seeFIG. 31 ). Internal helical ridges or screw threads 62 are formed withinthe bore 68. The internal screw threads 62 are sized and configured tomate with the complementary external screw threads 28 on the distalregion of the anchor body 12. The distal region of the anchor body 12can thereby be threaded to the anchor plate 58 (as shown in FIG. 32 ).When threaded to the anchor body 12, the anchor plate 58 rotates incommon with the anchor body 12 (as shown in FIG. 33 ).

Prior to introduction of the implant structure 20 into the broached bore48 formed in the manner previously described (and as shown in FIGS. 8Ato 8D), the anchor body 12 is passed through the bore 22 of the implantstructure 20, and the anchor plate 58 is threaded to the distal threadedregion 26 of the anchor body 12, which is sized to project beyond thedistal end of the implant structure 20. Further, as FIG. 32 shows, theanchor plate 58 is additionally rotationally oriented in a positionaligned with the distal end of the implant structure 20. In the alignedposition (FIG. 32 ), the apexes 64 of the anchor plate 58 overlay andregister with the apexes 66 of the distal end of the implant structure20. The implant structure 20, anchor body 12, and anchor plate 58 areintroduced as a unit through the broached bore 48 in the orientationshown in FIG. 32 . In the aligned position, the anchor plate 58 offersno resistance to passage of the implant structure 20 through thebroached bore 48.

Upon contacting the terminus of the broached bore, the proximal end ofthe anchor body 58 is rotated 60.degree. degrees (as shown in FIG. 33 ).The rotation moves the anchor plate 58 into an extended, bone-grippingposition no longer aligned with the distal end of the implant structure20 (as is shown in FIG. 33 ). In the extended, bone-gripping position,the apexes 64 of the triangular anchor plate 58 project radially outwardfrom the triangular sides 68 of the implant structure 20. The anchorplate 58 presents at the distal end of the implant structure 20 anenlarged lateral surface area, larger than the cross sectional area ofthe implant structure itself.

During rotation of the anchor plate 58 toward the bone-grippingposition, the cutting edges 72 of the anchor plate 58 advance into boneand cut bone, seating the anchor plate 58 into bone in the bone segmentor region (see FIG. 34 ). In the bone-gripping position, the anchorplate 58 anchors the distal end of the anchor body 12 into bone. Theanchor plate 58 resists axial migration and separation, in much the samefashion as the anchor screw 14.

The sides 68 of the implant structure 20 at the distal end of thestructure 20 preferably include cut-outs 70 (see FIGS. 31 and 32 ). Thecut-outs 70 are sized and configured so that, when the anchor plate 58is rotated into its bone-gripping position, the body of the anchor plate58 adjoining the apexes detents and comes to rest within the cut outs70, as FIG. 33 shows. Nested within the cut-outs 70, further tighteningof the anchor nut 16 and washer 18 at the proximal end of the anchorbody 12, as previously described, locks the anchor plate 58 in thebone-gripping, anchored position. By tightening the anchor nut, the moredistal end of the anchor body 12, anchored by the plate 58 in the secondbone segment, draws the second bone segment toward the first bonesegment, reducing the space or joint between them, while eventuallycompressing the implant structure 20 between the distal anchor plate 58and the proximal nut/washer (as FIG. 34 shows), thereby comprising acompression stem assembly 10.

2. Two Piece Compressible Implant Structure

An alternative embodiment of a compressible implant structure is shownin FIGS. 35A and 35B. In use, the implant structure is sized andconfigured to be implanted in adjoining bone segments, which areseparated by a space or joint, for the purpose of bone fixation or jointfusion, as already described.

In this embodiment (see FIG. 35A), the implant structure can possess acircular or curvilinear cross section, as previously described. Unlikeprevious implant structures, the implant structure 20 shown in FIG. 35Acomprises two mating implant components 74 and 78.

As before described, each implant component 74 and can be formed—e.g.,by machining, molding, or extrusion—from a durable material usable inthe prosthetic arts that is not subject to significant bio-absorption orresorption by surrounding bone or tissue over time.

Each implant component 74 and 78 includes exterior bony in-growth orthrough-growth regions, as previously described.

Prior to introduction of the implant structure, a broached bore isformed through the bone segments in the manner previously described, andis shown in FIGS. 8A to 8D. The implant component 74 is sized andconfigured to be securely affixed within the broached bore in the mostdistal bone segment where the broached bore terminates, e.g., by makingan interference fit and/or otherwise being secured by the use ofadhesives. The implant component 74 is intended to anchor the overallimplant structure.

The implant component 74 further includes a post 76 that extends throughthe broached bore into the most proximal bone segment, where thebroached bore originates. The post 76 includes internal threads 80.

The second implant component 78 is sized and configured to be introducedinto the broached bore of the most proximal bone segment. The secondimplant component includes an interior bore, so that the implantcomponent 78 is installed by sliding it over the post 76 of the firstimplant component 74, as FIG. 35B shows.

An anchor screw 16 (desirably with a washer 18) includes external screwthreads, which are sized and configured to mate with the complementaryinternal screw threads 80 within the post 76. Tightening the anchorscrew 16 draws the first and second implant components 74 and 78together, reducing the space or joint between the first and second bonesegments and putting the resulting implant structure into compression,as FIG. 35B shows.

3. Radial Compression

(Split Implant Structure)

An alternative embodiment of an implant structure 82 is shown in FIGS.36A and 36B. In use, the implant structure 82 is sized and configured tobe implanted in adjoining bone segments, which are separated by a spaceor joint, for the purpose of bone fixation or joint fusion, as alreadydescribed. The implant structure 82 is sized and configured to be placedinto radial compression.

The implant structure 82 includes a body that can possess a circular orcurvilinear cross section, as previously described. As before described,the implant structure 82 can be formed—e.g., by machining, molding, orextrusion—from a durable material usable in the prosthetic arts that isnot subject to significant bio-absorption or resorption by surroundingbone or tissue over time.

The implant structure 82 includes one or more exterior bony in-growth orthrough-growth regions, as previously described.

Unlike previously described implant structures, the proximal end of theimplant structure 82 includes an axial region of weakness comprising asplit 84. Further included is a self-tapping screw 16. The screw 16includes a tapered threaded body. The tapered body forms a wedge ofincreasing diameter in the direction toward the head of the screw 16.The screw 16 is self-tapping, being sized and configured to beprogressively advanced when rotated into the split 84, while creatingits own thread, as FIG. 36B shows.

Prior to introduction of the implant structure 84, a broached bore isformed through the bone segments in the manner previously described, andas shown in FIGS. 8A to 8D. The implant structure 84 is introduced intothe broached bore, as FIG. 36A shows. The implant structure is desirablysized and configured to be securely affixed within the broached bore inthe most distal bone segment where the broached bore terminates, e.g.,by making an interference fit and/or otherwise being secured by the useof adhesives. The interference fit and/or adhesives anchor the overallimplant structure 84.

After introduction of the implant structure 84 into the broached bore,the self-tapping screw 16 (desirably with a washer 18) is progressivelyadvanced by rotation into the split 84. The wedge-shape of the threadedbody of the screw 16 progressively urges the body of the implantstructure 84 to expand axially outward along the split 84, as FIG. 36Bshows. The expansion of the diameter of the body of the implantstructure 82 about the split 84 presses the proximal end of the implantstructure 82 into intimate contact against adjacent bone. The radialexpansion of the body of the implant structure 82 about the split 84radially compresses the proximal end of the implant structure 82 againstbone. The radial compression assures intimate contact between the bonyin-growth region and bone within the broached bore, as well as resistsboth rotational and axial migration of the implant structure 82 withinthe bone segments.

F. Implant Structures without Compression

It should be appreciated that an elongated, stem-like, implant structure20 having a bony in-growth and/or through-growth region, like that shownin FIG. 2 , can be sized and configured for the fixation of bonefractures (i.e., fixation of parts of the same bone) or for the fixationof bones which are to be fused (arthrodesed) throughout the body withoutassociation with a compression stem assembly 10 as just described, orwithout other means for achieving compression of the implant structureas just described. The configuration and use of representativeelongated, stem-like, implant structures 20 having bony in-growth and/orthrough-growth regions 24 for the fixation of bone fractures (i.e.,fixation of parts of the same bone) or for the fixation of bones whichare to be fused, without association with a compression stem assembly10, are described, e.g., in U.S. patent application Ser. No. 11/136,141,filed on May 24, 2005, titled “SYSTEMS AND METHODS FOR THE FIXATION ORFUSION OF BONE,” now U.S. Pat. No. 7,922,765 B2, which is incorporatedherein by reference.

II. Arthrodesis of the Sacroiliac Joint Using the Implant Structures

Elongated, stem-like implant structures 20 like that shown in FIG. 2(and the alternative embodiments) make possible the fixation of theSI-Joint (shown in anterior and posterior views, respectively, in FIGS.9 and 10 ) in a minimally invasive manner, with or without associationwith a compression stem assembly 10. These implant structures 20 can beeffectively implanted through the use of two alternative surgicalapproaches; namely, (i) a Lateral Approach, or (ii) a Postero-LateralApproach. Either procedure is desirably aided by conventional lateraland/or anterior-posterior (A-P) visualization techniques, e.g., usingX-ray image intensifiers such as a C-arms or fluoroscopes to produce alive image feed which is displayed on a TV screen.

A. The Lateral Approach

1. Without Association of a Compression Stem Assembly

In one embodiment of a lateral approach (see FIGS. 11, 12, and 13A/B),one or more implant structures 20 are introduced (without use of acompression stem assembly 10) laterally through the ilium, the SI-Joint,and into the sacrum S1. This path and resulting placement of the implantstructures 20 are best shown in FIGS. 12 and 13A/B. In the illustratedembodiment, three implant structures 20 are placed in this manner. Alsoin the illustrated embodiment, the implant structures 20 are triangularin cross section, but it should be appreciated that implant structures20 of other cross sections as previously described can be used.

Before undertaking a lateral implantation procedure, the physicianidentifies the SI-Joint segments that are to be fixated or fused(arthrodesed) using, e.g., the Faber Test, or CT-guided injection, orX-ray/MRI of SI Joint.

Aided by lateral and anterior-posterior (A-P) c-arms, and with thepatient lying in a prone position (on their stomach), the physicianaligns the greater sciatic notches (using lateral visualization) toprovide a true lateral position. A 3 cm incision is made startingaligned with the posterior cortex of the sacral canal, followed byblood-tissue separation to the ilium. From the lateral view, the guidepin 38 (with sleeve) (e.g., a Steinmann Pin) is started resting on theilium at a position inferior to the sacrum S1 end plate and justanterior to the sacral canal. In A-P and lateral views, the guide pin 38should be parallel to the S1 end plate at a shallow angle anterior(e.g., 15.degree. to 20.degree. off horizontal, as FIG. 13A shows). In alateral view, the guide pin 38 should be posterior to the sacrumanterior wall. In the A-P view, the guide pin 38 should be superior tothe S1 inferior foramen and lateral of mid-line. This correspondsgenerally to the sequence shown diagrammatically in FIGS. 8A and 8B. Asoft tissue protector (not shown) is desirably slipped over the guidepin 38 and firmly against the ilium before removing the guide pin 38sleeve.

Over the guide pin 38 (and through the soft tissue protector), the pilotbore 42 is drilled in the manner previously described, as isdiagrammatically shown in FIG. 8C. The pilot bore 42 extends through theilium, through the SI-Joint, and into the S1. The drill bit 40 isremoved.

The shaped broach 44 is tapped into the pilot bore 42 over the guide pin38 (and through the soft tissue protector) to create a broached bore 48with the desired profile for the implant structure 20, which, in theillustrated embodiment, is triangular. This generally corresponds to thesequence shown diagrammatically in FIG. 8D. The triangular profile ofthe broached bore 48 is also shown in FIG. 11 .

As shown in FIGS. 11 and 12 , a triangular implant structure 20 can benow tapped (in this embodiment, without an associated compression sleeveassembly) through the soft tissue protector over the guide pin 38through the ilium, across the SI-Joint, and into the S1, until theproximal end of the implant structure 20 is flush against the lateralwall of the ilium (see also FIGS. 13A and 13B). The guide pin 38 andsoft tissue protector are withdrawn, leaving the implant structure 20residing in the broached passageway, flush with the lateral wall of theilium (see FIGS. 13A and 13B). In the illustrated embodiment, twoadditional implant structures 20 are implanted in this manner, as FIG.12 best shows.

The implant structures 20 are sized according to the local anatomy. Forthe SI-Joint, representative implant structures 20 can range in size,depending upon the local anatomy, from about 35 mm to about 55 mm inlength, and about 7 mm diameter. The morphology of the local structurescan be generally understood by medical professionals using textbooks ofhuman skeletal anatomy along with their knowledge of the site and itsdisease or injury. The physician is also able to ascertain thedimensions of the implant structure 20 based upon prior analysis of themorphology of the targeted bone using, for example, plain film x-ray,fluoroscopic x-ray, or MRI or CT scanning.

2. With Association of a Compression Stem Assembly

As shown in FIGS. 14 to 16A/B, the lateral approach also lends itself tothe introduction of one or more implant structures 20 in associationwith compression stem assemblies 10, as previously described, laterallythrough the ilium, the SI-Joint, and into the sacrum S1. This path andresulting placement of the implant structures are best shown in FIGS.16A and 16B. As in the embodiment shown in FIGS. 11 to 13A/B, threeimplant structures 20 are placed in this manner. Also, as in theembodiment shown in FIGS. 11 to 13A/B, the implant structures aretriangular in cross section, but it still should be appreciated thatimplant structures having other cross sections, as previously described,can be used. In this embodiment of the lateral approach, the implantstructure 20 is not inserted immediately following the formation of thebroached bore 48. Instead, components of the compression stem assembly10 are installed first in the broached bore 48 to receive the implantstructure 20.

More particularly, following formation of the broached bore 48, aspreviously described, the guide pin 38 is removed, while keeping thesoft tissue protector in place. The anchor screw 14 of the compressionstem assembly 10 is seated in bone in the sacrum S beyond the terminusof the broached bore 48, in the manner generally shown in FIGS. 8E to8G. In this arrangement, to accommodate placement of the anchor screw 14of the compression stem assembly 10, an extent of bone in the sacrum S1is left native and undrilled beyond the terminus of the pilot bore 42and broached bore 48. The anchor screw 14 is advanced and buried in thisextent of native and undrilled bone in the sacrum S1, as FIGS. 16A and16B show, to be coupled to the threaded distal end 28 of the anchor body12.

The threaded proximal end 28 of the anchor body 12 is threaded into andmated to the anchor screw 14 within the sacrum S1, as previouslydescribed and as shown in FIG. 8H, with the remainder of the anchor body12 extending proximally through the SI-Joint and ilium, to project anexposed distance outside the lateral wall of the ilium, as FIGS. 16A and16B show. The implant structure 20 is then placed by sliding it over theanchor body 12, until flush against the lateral wall of the ilium, aspreviously described and as shown in FIG. 8 . The anchor washer 18 andnut are then installed and tightened on the proximal end of the anchorbody 12, as previously described and shown in FIGS. 8J to 8L, puttingthe assembly into compression. The resulting assembly is shown in FIGS.15 and 16A/B.

As shown in FIGS. 14 and 15 , three compression stem assemblies 10 canbe installed by lateral approach across the SI-Joint. As individualcompression stem assemblies are placed into compression by tighteningthe anchor nut 16, the implant structures of neighboring compressionstem assemblies may advance to project slightly beyond the lateral wallof the ilium. If this occurs, the projecting implant structures 20 canbe gently tapped further into the ilium over their respective anchorpins 12.

B. The Postero-Lateral Approach

1. Without Association of a Compression Stem Assembly

As shown in FIGS. 17 to 19A/B, one or more implant structures can beintroduced (without use of a compression stem assembly 10) in apostero-lateral approach entering from the posterior iliac spine of theilium, angling through the SI-Joint, and terminating in the sacral alae.This path and resulting placement of the implant structures 20 are bestshown in FIGS. 18 and 19A/B. In the illustrated embodiment, threeimplant structures 20 are placed in this manner. Also in the illustratedembodiment, the implant structures 20 are triangular in cross section,but it should be appreciated that implant structures 20 of other crosssections as previously described can be used.

The postero-lateral approach involves less soft tissue disruption thatthe lateral approach, because there is less soft tissue overlying theentry point of the posterior iliac spine of the ilium. Introduction ofthe implant structure 20 from this region therefore makes possible asmaller, more mobile incision. Further, the implant structure 20 passesthrough more bone along the postero-lateral route than in a strictlylateral route, thereby involving more surface area of the SI-Joint andresulting in more fusion and better fixation of the SI-Joint. Employingthe postero-lateral approach also makes it possible to bypass all nerveroots, including the L5 nerve root.

The set-up for a postero-lateral approach is generally the same as for alateral approach. It desirably involves the identification of theSI-Joint segments that are to be fixated or fused (arthrodesed) using,e.g., the Faber Test, or CT-guided injection, or X-ray/MRI of SI Joint.It is desirable performed with the patient lying in a prone position (ontheir stomach) and is aided by lateral and anterior-posterior (A-P)c-arms. The same surgical tools are used to form the pilot bore 42 overa guide pin 38, except the path of the pilot bore 42 now starts from theposterior iliac spine of the ilium, angles through the SI-Joint, andterminates in the sacral alae. The pilot bore 42 is shaped into thedesired profile using a broach, as before described (shown in FIG. 17 ),and the implant structure 20 is inserted into the broached bore 48 themanner shown in FIGS. 18 and 19A/B. The triangular implant structure 20is tapped (in this embodiment, without an associated compression sleeveassembly 10) through the soft tissue protector over the guide pin 38from the posterior iliac spine of the ilium, angling through theSI-Joint, and terminating in the sacral alae, until the proximal end ofthe implant structure 20 is flush against the posterior iliac spine ofthe ilium, as FIG. 18 shows. As shown in FIGS. 17 to 19A/B, threeimplant structures 20 are introduced in this manner. Because of theanatomic morphology of the bone along the postero-lateral route, it maybe advisable to introduce implant structures of difference sizes, withthe most superior being the longest in length, and the others beingsmaller in length.

2. With Association of a Compression Stem Assembly

As shown in FIGS. 20 to 22A/B, the postero-lateral approach also lendsitself to the introduction of one or more implant structures 20 inassociation with compression stem assemblies 10, as previouslydescribed, entering from the posterior iliac spine of the ilium, anglingthrough the SI-Joint, and advancing into the sacral alae. This path andresulting placement of the implant structures 20 with compression stemassemblies 10 are best shown in FIGS. 22A/B. As in the embodiment shownin FIGS. 17 to 19A/B, three implant structures 20 are placed inthis-manner. Also, as in the embodiment shown in FIGS. 17 to 19A/B, theimplant structures 20 are triangular in cross section, but it stillshould be appreciated that implant structures 20 of other cross sectionsas previously described can be used. In this embodiment of theposterior-lateral approach, the implant structure 20 is not insertedimmediately following the formation of the broached bore 48. Instead,components of the compression stem assembly 10 are installed in thebroached bore 48 first to receive the implant structure 20, as have beenpreviously described as is shown in FIG. 20 .

As before explained, the set-up for a postero-lateral approach isgenerally the same as for a lateral approach. It is desirable performedwith the patient lying in a prone position (on their stomach) and isaided by lateral and anterior-posterior (A-P) c-arms. The same surgicaltools are used to form the pilot bore 42 over a guide pin 38 that startsfrom the posterior iliac spine of the ilium, angles through theSI-Joint, and terminates in the sacral alae. The pilot bore 42 is shapedinto the desired profile using a broach 44, as before described (and asshown in FIG. 20 ). In this arrangement, to accommodate placement of theanchor screw 14 of the compression stem assembly 10, an extent of bonein the sacral alae is left native and undrilled beyond the terminus ofthe formed pilot bore 42 and broached bore 48. The anchor screw 14 isadvanced and buried in this extent of native and undrilled bone in thesacral alae, as FIGS. 22A/B show, to be coupled to the threaded distalend 28 of the anchor body 12. Due to the morphology of the sacral alae,the anchor screw 14 may be shorter than it would be if buried in thesacrum S1 by the lateral approach.

The threaded proximal end 28 of the anchor body 12 is threaded into andmated to the anchor screw 14 within the sacral alae, as previouslydescribed and as shown in FIG. 8H, with the remainder of the anchor body12 extending proximally through the SI-Joint to project an exposeddistance outside the superior iliac spine of the ilium, as FIGS. 21 to22A/B show. The implant structure 20 is then placed by sliding it overthe anchor body 12, until flush against the superior iliac spine of theilium, as previously described and as shown in FIG. 8I. The anchorwasher 18 and nut are then installed and tightened on the proximal endof the anchor body 12, as previously described and shown in FIGS. 8J to8L, putting the assembly 10 into compression. The resulting assembly 10is shown in FIGS. 21 and 22A/B.

As shown in FIGS. 20 and 21 , three compression stem assemblies 10 canbe installed by postero-lateral approach across the SI-Joint. As beforeexplained, as individual compression stem assemblies 10 are placed intocompression by tightening the anchor nut 16, the implant structures 20of neighboring compression stem assemblies 10 may advance to projectslightly beyond the superior iliac spine of the ilium. If this occurs,the projecting implant structures 20 can be gently tapped further intothe superior iliac spine of the ilium over their respective anchorbodies 12.

C. Conclusion

Using either a posterior approach or a postero-lateral approach, one ormore implant structures 20 can be individually inserted in a minimallyinvasive fashion, with or without association of compression stemassemblies 10, or combinations thereof, across the SI-Joint, as has beendescribed. Conventional tissue access tools, obturators, cannulas,and/or drills can be used for this purpose. No joint preparation,removal of cartilage, or scraping are required before formation of theinsertion path or insertion of the implant structures 20, so a minimallyinvasive insertion path sized approximately at or about the maximumouter diameter of the implant structures 20 need be formed.

The implant structures 20, with or without association of compressionstem assemblies 10, obviate the need for autologous bone graft material,additional pedicle screws and/or rods, hollow modular anchorage screws,cannulated compression screws, threaded cages within the joint, orfracture fixation screws.

In a representative procedure, one to six, or perhaps eight, implantstructures 20 might be needed, depending on the size of the patient andthe size of the implant structures 20. After installation, the patientwould be advised to prevent loading of the SI-Joint while fusion occurs.This could be a six to twelve week period or more, depending on thehealth of the patient and his or her adherence to post-op protocol.

The implant structures 20 make possible surgical techniques that areless invasive than traditional open surgery with no extensive softtissue stripping. The lateral approach and the postero-lateral approachto the SI-Joint provide straightforward surgical approaches thatcomplement the minimally invasive surgical techniques. The profile anddesign of the implant structures 20 minimize rotation and micromotion.Rigid implant structures 20 made from titanium provide immediate post-opSI Joint stability. A bony in-growth region 24 comprising a porousplasma spray coating with irregular surface supports stable bonefixation/fusion. The implant structures 20 and surgical approaches makepossible the placement of larger fusion surface areas designed tomaximize post-surgical weight bearing capacity and provide abiomechanically rigorous implant designed specifically to stabilize theheavily loaded SI-Joint.

III. Arthrodesis of the Sacroiliac Joint Using Other Structures

The Lateral Approach and the Postero-Lateral Approach to the SI-Joint,aided by conventional lateral and/or anterior-posterior (A-P)visualization techniques, make possible the fixation of the SI-Joint ina minimally invasive manner using other forms of fixation/fusionstructures. Either approach makes possible minimal incision size, withminimal soft tissue stripping, minimal tendon irritation, less pain,reduced risk of infection and complications, and minimal blood loss.

For example (see FIGS. 23 and 24A/B, one or more screw-like structures52, e.g., a hollow modular anchorage screw, or a cannulated compressionscrew, or a fracture fixation screw, can be introduced using the lateralapproach described herein, being placed laterally through the ilium, theSI-Joint, and into the sacrum S1. This path and resulting placement ofthe screw-like structures 52 are shown in FIGS. 23 and 24A/B. Desirably,the screw-like structure carry a bony in-growth material or a bonythrough-growth configuration, as described, as well as being sized andconfigured to resist rotation after implantation.

Likewise, one or more of the screw-like structures 52 can be introducedusing the postero-lateral approach described herein, entering from theposterior iliac spine of the ilium, angling through the SI-Joint, andterminating in the sacral alae. This path and resulting placement of thescrew-like structure are shown in FIGS. 25 and 26A/B. Desirably, thescrew-like structures 52 carry a bony in-growth material or a bonythrough-growth configuration, as described, as well as being sized andconfigured to resist rotation after implantation, as before described.

As another example, one or more fusion cage structures 54 containingbone graft material can be introduced using the lateral approachdescribed herein, being placed laterally through the ilium, theSI-Joint, and into the sacrum S1. This path and resulting placement ofthe fusion cage structures 54 are shown in FIGS. 27 and 28A/B. Such astructure 54 may include an anchor screw component 56, to be seated inthe sacrum S1, as shown in FIGS. 27 and 28A/B.

Likewise, one or more of the fusion cage structures 54 can be introducedusing the postero-lateral approach described herein, entering from theposterior iliac spine of the ilium, angling through the SI-Joint, andterminating in the sacral alae. This path and resulting placement of thefusion cage structures 54 are shown in FIGS. 29 and 30A/B. Such astructure 54 may include an anchor screw component 56, to be seated inthe sacral alae, as shown in FIGS. 27 and 28A/B.

IV. Conclusion

The foregoing is considered as illustrative only of the principles ofthe invention. Furthermore, since numerous modifications and changeswill readily occur to those skilled in the art, it is not desired tolimit the invention to the exact construction and operation shown anddescribed. While the preferred embodiment has been described, thedetails may be changed without departing from the invention, which isdefined by the claims.

The foregoing is considered as illustrative only of the principles ofthe invention. Furthermore, since numerous modifications and changeswill readily occur to those skilled in the art, it is not desired tolimit the invention to the exact construction and operation shown anddescribed. While the preferred embodiment has been described, thedetails may be changed without departing from the invention, which isdefined by the claims.

Part II

The following describes embodiments of the implant for the fusion orfixation of other joints or bone segments.

I. The Implant Structure

FIG. 40 shows a representative embodiment of an elongated, stem-like,cannulated implant structure 20. As will be described in greater detaillater, the implant structure 20 is sized and configured for the fixationof bones which are to be fused (arthrodesed) (i.e. fixation of two ormore individual bones that are adjacent and/or jointed) and/or thestabilization of adjacent bone structures. In particular, and as will bedemonstrated, the implant structure is well suited for the fusion orstabilization of adjacent bone structures in the lumbar region of thespine, either across the intervertebral disc or across one or more facetjoints.

The implant structure 20 can be formed—e.g., by machining, molding, orextrusion—from a durable material usable in the prosthetic arts that isnot subject to significant bio-absorption or resorption by surroundingbone or tissue over time. The implant structure 20, is intended toremain in place for a time sufficient to stabilize a bone fracture orfusion site. Such materials include, but are not limited to, titanium,titanium alloys, tantalum, tivanium (aluminum, vanadium, and titanium),chrome cobalt, surgical steel, or any other total joint replacementmetal and/or ceramic, sintered glass, artificial bone, any uncementedmetal or ceramic surface, or a combination thereof.

Alternatively, the implant structure 20 may be formed from a suitabledurable biologic material or a combination of metal and biologicmaterial, such as a biocompatible bone-filling material. The implantstructure 20 may be molded from a flowable biologic material, e.g.,acrylic bone cement, that is cured, e.g., by UV light, to a non-flowableor solid material.

The implant structure 20 is sized according to the local anatomy. Themorphology of the local structures can be generally understood bymedical professionals using textbooks of human skeletal anatomy alongwith their knowledge of the site and its disease or injury. Thephysician is also able to ascertain the dimensions of the implantstructure 20 based upon prior analysis of the morphology of the targetedbone region using, for example, plain film x-ray, fluoroscopic x-ray, orMRI or CT scanning.

As FIGS. 41 to 44 show, the implant structure 20 can take various shapesand have various cross-sectional geometries. The implant structure 20can have, e.g., a generally curvilinear (i.e., round or oval)cross-section—as FIG. 41 shows for purposes of illustration—or agenerally rectilinear cross section (i.e., square or rectangular orhexagon or H-shaped or triangular—as FIG. 42 shows for purposes ofillustration—or combinations thereof. In FIG. 40 , the implant structure20 is shown to be triangular in cross section, which effectively resistsrotation and micromotion once implanted.

As FIGS. 43 and 44 show, the implant structure 20, whether curvilinear(FIG. 43 ) or rectilinear (FIG. 44 ) can include a tapered region 34 atleast along a portion of its axial length, meaning that the width ordiameter of the implant structure 20 incrementally increases along itsaxial length. Desirably, the tapered region 34 corresponds with, in use,the proximal region of the implant structure 20 (i.e., the last part ofthe implant structure 20 to enter bone). The amount of the incrementalincrease in width or diameter can vary. As an example, for an implantstructure 20 having a normal diameter of 7 mm, the magnitude of theincremental increase at its maximum can range between about 0.25 mm to1.25 mm. The tapered region 34 enhances the creation and maintenance ofcompression between bone segments or regions.

As FIG. 40 shows, the implant structure 20 includes a region 24 formedalong at least a portion of its length to promote bony in-growth onto orinto surface of the structure and/or bony growth entirely through all ora portion of the structure. The bony in-growth or through-growth region24 along the surface of the implant structure 20 accelerates bonyin-growth or through-growth onto, into, or through the implant structure20. Bony in-growth or through-growth onto, into, or through the implantstructure 20 helps speed up the fusion process of the adjacent boneregions fixated by the implant structure 20.

The bony in-growth or through-growth region 24 desirably extends alongthe entire outer surface of the implant structure 20, as shown in FIGS.40 to 44 . The bony in-growth region 24 or through-growth can comprise,e.g., through holes, and/or various surface patterns, and/or varioussurface textures, and/or pores, or combinations thereof. Theconfiguration of the bony in-growth or through-growth region 24 can, ofcourse, vary. By way of examples, the bony in-growth or through-growthregion 24 can comprise an open mesh configuration; or beadedconfiguration; or a trabecular configuration; or include holes orfenestrations. Any configuration conducive to bony in-growth and/or bonythrough-growth will suffice.

The bony in-growth or through-growth region 24 can be coated or wrappedor surfaced treated to provide the bony in-growth or through-growthregion, or it can be formed from a material that itself inherentlypossesses a structure conducive to bony in-growth or through-growth,such as a porous mesh, hydroxyapetite, or other porous surface. The bonyin-growth or through-growth region can includes holes that allow bone togrow throughout the region.

In a preferred embodiment, the bony in-growth region or through-growthregion 24 comprises a porous plasma spray coating on the implantstructure 20. This creates a biomechanically rigorous fixation/fusionsystem, designed to support reliable fixation/fusion and acute weightbearing capacity.

The bony in-growth or through-growth region 24 may further be coveredwith various other coatings such as antimicrobial, antithrombotic, andosteoinductive agents, or a combination thereof. The entire implantstructure 20 may be impregnated with such agents, if desired.

The implant structure includes an interior bore that accommodates itsplacement in a non-invasive manner by sliding over a guide pin, as willbe described in greater detail later.

As before stated, the implant structure 20 is well suited for the fusionand/or stabilization of adjacent bone structures in the lumbar region ofthe spine. Representative examples of the placement of the implantstructure 20 in the lumbar region of the spine will now be described.

A. Use of the Implant Structures to Achieve Anterior Lumbar InterbodyFusion

FIG. 45 shows, in an exploded view prior to implantation, arepresentative configuration of an assembly of one or more implantstructures 20 sized and configured to achieve anterior lumbar interbodyfusion, in a non-invasive manner and without removal of theintervertebral disc. FIGS. 46 to 48 show the assembly afterimplantation, respectively, in an anterior view, a right lateral view,and a superior left lateral perspective view.

In the representative embodiment illustrated in FIGS. 46 to 48 , theassembly comprises three implant structures 20. It should beappreciated, however, that a given assembly can include a greater orlesser number of implant structures 20.

In the representative embodiment shown in FIGS. 46 to 48 , the threeimplant structures 20 are spaced in an adjacent lateral array. Theimplant structures 20 extend from an anterolateral region of a selectedvertebral body (i.e., a lateral region anterior to a transverseprocess), across the intervertebral disc into an opposite anterolateralregion of an adjacent caudal (inferior) vertebra. As shown in FIGS. 46to 48 , the array of implant structures 20 extends in an angled path(e.g., about 20.degree. to about 40.degree. off horizontal) through thecranial (superior) lumbar vertebral body (shown as L4) in an inferiordirection, through the adjoining intervertebral disc, and terminates inthe next adjacent caudal (inferior) lumbar vertebral body (shown as L5).

More particularly, in the representative embodiment shown in FIGS. 45 to48 , the implant structures 20 enter the right anterolateral region ofvertebra L4 and terminate within the left anterolateral interior ofvertebra L5, spanning the intervertebral disc between L4 and L5.

Alternatively, or in combination, an array of implant structures 20 canlikewise extend between L5 and S1 in the same trans-disc formation.

The implant structures 20 are sized according to the local anatomy. Theimplant structures 20 can be sized differently, e.g., 3 mm, 4 mm, 6 mm,etc.), to accommodate anterolateral variations in the anatomy. Theimplant structures 20 can be sized for implantation in adults orchildren.

The intimate contact created between the bony in-growth orthrough-growth region 24 along the surface of the implant structure 20accelerates bony in-growth or through-growth onto, into, or through theimplant structure 20, to accelerate trans-disc fusion between theselumbar vertebrae.

FIGS. 49A to 49G diagrammatically show, for purposes of illustration, arepresentative lateral (or posterolateral) procedure for implanting theassembly of implant structures 20 shown in FIGS. 46 to 48 .

The physician identifies the vertebrae of the lumbar spine region thatare to be fused using, e.g., the Faber Test, or CT-guided injection, orX-ray/MRI of the lumbar spine. Aided by lateral and anterior-posterior(A-P) c-arms, and with the patient lying in a prone position (on theirstomach), the physician makes a 3 mm incision laterally orposterolaterally from the side (see FIG. 49A). Aided by conventionalvisualization techniques, e.g., using X-ray image intensifiers such as aC-arms or fluoroscopes to produce a live image feed which is displayedon a TV screen, a guide pin 38 is introduced by conventional means intoL4 (see FIG. 49B) for the first, most anterolateral implant structure(closest to the right transverse process of L4), in the desired angledinferiorly-directed path through the intervertebral disc and into theinterior left anterolateral region of vertebra L5.

When the guide pin 38 is placed in the desired orientation, thephysician desirable slides a soft tissue protector over the guide pin 38before proceeding further. To simplify the illustration, the soft tissueprotector is not shown in the drawings.

Through the soft tissue protector, a cannulated drill bit 40 is nextpassed over the guide pin 38 (see FIG. 49C). The cannulated drill bit 40forms a pilot insertion path or bore 42 along the first angled pathdefined by the guide pin 38. A single drill bit or multiple drill bits40 can be employed to drill through bone fragments or bone surfaces tocreate a pilot bore 42 of the desired size and configuration.

When the pilot bore 42 is completed, the cannulated drill bit 40 iswithdrawn over the guide pin 38.

Through the soft tissue protector, a broach 44 having the externalgeometry and dimensions matching the external geometry and dimensions ofthe implant structure 20 (which, in the illustrated embodiment, istriangular) (see FIG. 49D) is tapped through the soft tissue protectorover the guide pin 38 and into the pilot bore 42. The shaped broach 44cuts along the edges of the pilot bore 42 to form the desired profile(which, in the illustrated embodiment, is triangular) to accommodate theimplant structure 20.

The broach 44 is withdrawn (see FIG. 49E), and the first, mostanterolateral implant structure 20 is passed over the guide pin 38through the soft tissue protector into the broached bore 48. The guidepin 38 and soft tissue protector are withdrawn from the first implantstructure 20.

The physician repeats the above-described procedure sequentially for thenext anterolateral implant structures 20: for each implant structure,inserting the guide pin 38, forming the pilot bore, forming the broachedbore, inserting the respective implant structure, withdrawing the guidepin, and then repeating the procedure for the next implant structure,and so on until all implant structures 20 are placed (as FIGS. 49F and49G indicate). The incision site(s) are closed.

In summary, the method for implanting the assembly of the implantstructures 20 comprises (i) identifying the bone structures to be fusedand/or stabilized; (ii) opening an incision; (iii) using a guide pin toestablished a desired implantation path through bone for the implantstructure 20; (iv) guided by the guide pin, increasing the cross sectionof the path; (v) guided by the guide pin, shaping the cross section ofthe path to correspond with the cross section of the implant structure20; (vi) inserting the implant structure 20 through the path over theguide pin; (vii) withdrawing the guide pin; (viii) repeating, asnecessary, the procedure sequentially for the next implant structure(s)until all implant structures 20 contemplated are implanted; and (ix)closing the incision.

As FIGS. 50 and 51 show, assemblies comprising one or more implantstructures 20 can be inserted from left and/or right anterolateralregions of a given lumbar vertebra, in an angled path through theintervertebral disc and into an opposite anterolateral interior regionof the next inferior lumbar vertebra.

For purposes of illustration, FIG. 50 shows two implant structures 20entering on the right anterolateral side of L4, through theintervertebral disc and into the left anterolateral region of L5, andone implant structure 20 entering on the left anterolateral side of L4,through the intervertebral disc and into the right anterolateral regionof L5. In this arrangement, the left and right implant structures 20cross each other in transit through the intervertebral disc.

As another illustration of a representative embodiment, FIG. 51 showsone implant structure 20 entering on the right anterolateral side of L4,through the intervertebral disc and into the left anterolateral regionof L5, and one implant structure 20 entering on the left anterolateralside of L4, through the intervertebral disc and into the rightanterolateral region of L5. In this arrangement as well, the left andright implant structures 20 cross each other in transit through theintervertebral disc.

B. Use of Implant Structures to Achieve Translaminal Lumbar Fusion(Posterior Approach)

FIG. 52 shows, in an exploded view prior to implantation, arepresentative configuration of an assembly of one or more implantstructures 20 sized and configured to achieve translaminar lumbar fusionin a non-invasive manner and without removal of the intervertebral disc.FIG. 53 shows the assembly after implantation, respectively, in aninferior transverse plane view.

As can be seen in the representative embodiment illustrated in FIGS. 52and 53 , the assembly comprises two implant structures 20. The firstimplant structure 20 extends from the left superior articular process ofvertebra L5, through the adjoining facet capsule into the left inferiorarticular process of vertebra L4, and, from there, further through thelamina of vertebra L4 into an interior right posterolateral region ofvertebra L4 adjacent the spinous process. The second implant structure20 extends from the right superior articular process of vertebra L5,through the adjoining facet capsule into the right inferior articularprocess of vertebra L4, and, from there, further through the lamina ofvertebra L4 into an interior left posterolateral region of vertebra L4adjacent the spinous process. The first and second implant structures 20cross each other within the medial lamina of vertebra L4.

The first and second implant structures 20 are sized and configuredaccording to the local anatomy. The selection of a translaminar lumbarfusion (posterior approach) is indicated when the facet joints arealigned with the sagittal plane. Removal of the intervertebral disc isnot required, unless the condition of the disc warrants its removal.

A procedure incorporating the technical features of the procedure shownin FIGS. 49A to 49G can be tailored to a posterior procedure forimplanting the assembly of implant structures 20 shown in FIGS. 52 and53 . The method comprises (i) identifying the vertebrae of the lumbarspine region that are to be fused; (ii) opening an incision, whichcomprises, e.g., with the patient lying in a prone position (on theirstomach), making a 3 mm posterior incision; and (iii) using a guide pinto established a desired implantation path through bone for the first(e.g., left side) implant structure 20, which, in FIGS. 52 and 53 ,traverses through the left superior articular process of vertebra L5,through the adjoining facet capsule into the left inferior articularprocess of vertebra L4, and then through the lamina of vertebra L4 intoan interior right posterolateral region of vertebra L4 adjacent thespinous process. The method further includes (iv) guided by the guidepin, increasing the cross section of the path; (v) guided by the guidepin, shaping the cross section of the path to correspond with the crosssection of the implant structure; (vi) inserting the implant structure20 through the path over the guide pin; (vii) withdrawing the guide pin;and (viii) using a guide pin to established a desired implantation paththrough bone for the second (e.g., right side) implant structure 20,which, in FIGS. 52 and 53 , traverses through the right superiorarticular process of vertebra L5, through the adjoining facet capsuleinto the right inferior articular process of vertebra L4, and throughthe lamina of vertebra L4 into an interior left posterolateral region ofvertebra L4 adjacent the spinous process. The physician repeats theremainder of the above-described procedure sequentially for the rightimplant structure 20 as for the left, and, after withdrawing the guidepin, closes the incision.

The intimate contact created between the bony in-growth orthrough-growth region 24 along the surface of the implant structure 20across the facet joint accelerates bony in-growth or through-growthonto, into, or through the implant structure 20, to accelerate fusion ofthe facets joints between L4 and L5. Of course, translaminar lumbarfusion between L5 and S1 can be achieved using first and second implantstructures in the same manner.

C. Use of Implant Structures to Achieve Lumbar Facet Fusion (PosteriorApproach)

FIG. 54 shows, in an exploded view prior to implantation, arepresentative configuration of an assembly of one or more implantstructures 20 sized and configured to lumbar facet fusion, in anon-invasive manner and without removal of the intervertebral disc.FIGS. 55 and 56 show the assembly after implantation, respectively, inan inferior transverse plane view and a lateral view.

As can be seen in the representative embodiment illustrated in FIGS. 54and 56 , the assembly comprises two implant structures 20. The firstimplant structure 20 extends from the left inferior articular process ofvertebra L4, through the adjoining facet capsule into the left superiorarticular process of vertebra L5 and into the pedicle of vertebra L5.The second implant structure 20 extends from the right inferiorarticular process of vertebra L5, through the adjoining facet capsuleinto the right superior articular process of vertebra L5 and into thepedicle of vertebra L5. In this arrangement, the first and secondimplant structures 20 extend in parallel directions on the left andright pedicles of vertebra L5. The first and second implant structures20 are sized and configured according to the local anatomy. Theselection of lumbar facet fusion (posterior approach) is indicated whenthe facet joints are coronally angled. Removal of the intervertebraldisc is not necessary, unless the condition of the disc warrants itsremoval.

A procedure incorporating the technical features of the procedure shownin FIGS. 49A to 49G can be tailored to a posterior procedure forimplanting the assembly of implant structures 20 shown in FIGS. 54 to 56. The method comprises (i) identifying the vertebrae of the lumbar spineregion that are to be fused; (ii) opening an incision, which comprises,e.g., with the patient lying in a prone position (on their stomach),making a 3 mm posterior incision; and (iii) using a guide pin toestablished a desired implantation path through bone for the first(e.g., left side) implant structure 20, which, in FIGS. 54 to 56 ,traverses through the left inferior articular process of vertebra L4,through the adjoining facet capsule into the left superior articularprocess of vertebra L5 and into the pedicle of vertebra L5. The methodfurther includes (iv) guided by the guide pin, increasing the crosssection of the path; (v) guided by the guide pin, shaping the crosssection of the path to correspond with the cross section of the implantstructure 20; (vi) inserting the implant structure 20 through the pathover the guide pin; (vii) withdrawing the guide pin; and (viii) using aguide pin to established a desired implantation path through bone forthe second (e.g., right side) implant structure 20, which, in FIGS. 54to 56 , traverses through the right inferior articular process ofvertebra L5, through the adjoining facet capsule into the right superiorarticular process of vertebra L5 and into the pedicle of vertebra L5.The physician repeats the remainder of the above-described proceduresequentially for the right implant structure 20 as for the left and,withdrawing the guide pin, closes the incision.

The intimate contact created between the bony in-growth orthrough-growth region 24 along the surface of the implant structure 20across the facet joint accelerates bony in-growth or through-growthonto, into, or through the implant structure 20, to accelerate fusion ofthe facets joints between L4 and L5.

Of course, translaminar lumbar fusion between L5 and S1 can be achievedusing first and second implant structures in the same manner.

D. Use of Implant Structures to Achieve Trans-Iliac Lumbar Fusion(Anterior Approach)

FIG. 57A shows, in an exploded view prior to implantation, arepresentative configuration of an assembly of one or more implantstructures 20 sized and configured to achieve fusion between lumbarvertebra L5 and sacral vertebra S1, in a non-invasive manner and withoutremoval of the intervertebral disc. FIG. 57B shows the assembly afterimplantation.

In the representative embodiment illustrated in FIGS. 57A and 57B, theassembly comprises two implant structures 20. It should be appreciated,however, that a given assembly can include a greater or lesser number ofimplant structures 20.

As FIGS. 57A and 57B show, the assembly comprises two implant structures20 inserted from left and right anterolateral regions of lumbar vertebraL5, in an angled path (e.g., about 20.degree. to about 40.degree. offhorizontal) through the intervertebral disc in an inferior direction,into and through opposite anterolateral interior regions of sacralvertebra S1, through the sacro-iliac joint, and terminating in theilium. In this arrangement, the left and right implant structures 20cross each other in transit through the intervertebral disc. As beforedescribed, the implant structures 20 are sized according to the localanatomy.

The intimate contact created between the bony in-growth orthrough-growth region 24 along the surface of the implant structure 20accelerates bony in-growth or through-growth onto, into, or through theimplant structure 20, to accelerate lumbar trans-iliac fusion betweenvertebra L5 and S1.

A physician can employ the lateral (or posterolateral) procedure asgenerally shown in FIGS. 49A to 49G for implanting the assembly ofimplant structures 20 shown in FIGS. 57A and 57B, including forming apilot bore over a guide pin inserted in the angled path, forming abroached bore, inserting the right implant 20 structure, withdrawing theguide pin, and repeating for the left implant structure 20, or viceversa. The incision site(s) are closed.

The assembly as described makes possible the achievement of trans-iliaclumbar fusion using an anterior in a non-invasive manner, with minimalincision, and without necessarily removing the intervertebral discbetween L5 and S1.

E. Use of Implant Structures to Achieve Trans-Iliac Lumbar Fusion(Postero-Lateral Approach from Posterior Iliac Spine)

FIG. 58A shows, in an exploded view prior to implantation, anotherrepresentative configuration of an assembly of one or more implantstructures 20 sized and configured to achieve fusion between lumbarvertebra L5 and sacral vertebra S1, in a non-invasive manner and withoutremoval of the intervertebral disc. FIGS. 58B and 58C show the assemblyafter implantation.

As FIGS. 58A and 58B show, the one or more implant structures areintroduced in a postero-lateral approach entering from the posterioriliac spine of the ilium, angling through the SI-Joint into and throughthe sacral vertebra S1, and terminating in the lumbar vertebra L5. Thispath and resulting placement of the implant structures 20 are also shownin FIG. 58C. In the illustrated embodiment, two implant structures 20are placed in this manner, but there can be more or fewer implantstructures 20. Also in the illustrated embodiment, the implantstructures 20 are triangular in cross section, but it should beappreciated that implant structures 20 of other cross sections aspreviously described can be used.

The postero-lateral approach involves less soft tissue disruption thatthe lateral approach, because there is less soft tissue overlying theentry point of the posterior iliac spine of the ilium. Introduction ofthe implant structure 20 from this region therefore makes possible asmaller, more mobile incision.

The set-up for a postero-lateral approach is generally the same as for alateral approach. It desirably involves the identification of the lumbarregion that is to be fixated or fused (arthrodesed) using, e.g., theFaber Test, or CT-guided injection, or X-ray/MRI of SI Joint. It isdesirable performed with the patient lying in a prone position (on theirstomach) and is aided by lateral and anterior-posterior (A-P) c-arms.The same surgical tools are used to form the pilot bore over a guide pin(e.g., on the right side), except the path of the pilot bore now startsfrom the posterior iliac spine of the ilium, angles through theSI-Joint, and terminates in the lumbar vertebra L5. The broached bore isformed, and the right implant 20 structure is inserted. The guide pin iswithdrawn, and the procedure is repeated for the left implant structure20, or vice versa. The incision site(s) are closed.

The assembly as described makes possible the achievement of trans-iliaclumbar fusion using a postero-lateral approach in a non-invasive manner,with minimal incision, and without necessarily removing theintervertebral disc between L5 and S1.

F. Use of Implant Structures to Stabilize a Spondylolisthesis

FIG. 59 shows a spondylolisthesis at the L5/S1 articulation, in whichthe lumbar vertebra L5 is displaced forward (anterior) of the sacralvertebra S1. As FIG. 59 shows, the posterior fragment of L5 remains innormal relation to the sacrum, but the anterior fragment and the L5vertebral body has moved anteriorly. Spondylolisthesis at the L5/S1articulation can result in pressure in the spinal nerves of the caudaequine as they pass into the superior part of the sacrum, causing backand lower limb pain.

FIG. 60A shows, in an exploded view prior to implantation, arepresentative configuration of an assembly of one or more implantstructures 20 sized and configured to stabilize the spondylolisthesis atthe L5/S1 articulation. FIGS. 60B and 60C show the assembly afterimplantation.

As shown, the implant structure 20 extends from a posterolateral regionof the sacral vertebra S1, across the intervertebral disc into anopposite anterolateral region of the lumbar vertebra L5. The implantstructure 20 extends in an angled path (e.g., about 20.degree. to about40.degree. off horizontal) through the sacral vertebra S1 in a superiordirection, through the adjoining intervertebral disc, and terminates inthe lumbar vertebra L5.

A physician can employ a posterior approach for implanting the implantstructure 20 shown in FIGS. 60A, 60B, and 60C, which includes forming apilot bore over a guide pin inserted in the angled path from theposterior of the sacral vertebra S1 through the intervertebral disc andinto an opposite anterolateral region of the lumbar vertebra L5, forminga broached bore, inserting the implant structure 20, and withdrawing theguide pin. The incision site is then closed. As previously described,more than one implant structure 20 can be placed in the same manner tostabilize a spondylolisthesis. Furthermore, a physician can fixate theimplant structure(s) 20 using the anterior trans-iliac lumbar path, asshown in FIG. 57A/B or 58A/B/C.

The physician can, if desired, combine stabilization of thespondylolisthesis, as shown in FIGS. 60A/B/C, with a reduction,realigning L5 and S-1. The physician can also, if desired, combinestabilization of the spondylolisthesis, as shown in FIGS. 60 A/B/C (withor without reduction of the spondylolisthesis), with a lumbar facetfusion, as shown in FIGS. 54 to 56 . The physician can also, if desired,combine stabilization of the spondylolisthesis, as shown in FIGS.60A/B/C, with a decompression, e.g., by the posterior removal of thespinous process and laminae bilaterally.

II. Conclusion

The various representative embodiments of the assemblies of the implantstructures 20, as described, make possible the achievement of diverseinterventions involving the fusion and/or stabilization of lumbar andsacral vertebra in a non-invasive manner, with minimal incision, andwithout the necessitating the removing the intervertebral disc. Therepresentative lumbar spine interventions described can be performed onadults or children and include, but are not limited to, lumbar interbodyfusion; translaminar lumbar fusion; lumbar facet fusion; trans-iliaclumbar fusion; and the stabilization of a spondylolisthesis. It shouldbe appreciated that such interventions can be used in combination witheach other and in combination with conventional fusion/fixationtechniques to achieve the desired therapeutic objectives.

Significantly, the various assemblies of the implant structures 20 asdescribed make possible lumbar interbody fusion without the necessity ofremoving the intervertebral disc. For example, in conventional anteriorlumbar interbody fusion procedures, the removal of the intervertebraldisc is a prerequisite of the procedure. However, when using theassemblies as described to achieve anterior lumbar interbody fusion,whether or not the intervertebral disc is removed depends upon thecondition of the disc, and is not a prerequisite of the procedureitself. If the disc is healthy and has not appreciably degenerated, oneor more implant structures 20 can be individually inserted in aminimally invasive fashion, across the intervertebral disc in the lumbarspine area, leaving the disc intact.

In all the representative interventions described, the removal of adisc, or the scraping of a disc, is at the physician's discretion, basedupon the condition of the disc itself, and is not dictated by theprocedure.

The bony in-growth or through-growth regions 24 of the implantstructures 20 described provide both extra-articular and intra osseousfixation, when bone grows in and around the bony in-growth orthrough-growth regions 24.

Conventional tissue access tools, obturators, cannulas, and/or drillscan be used during their implantation. No disc preparation, removal ofbone or cartilage, or scraping are required before and during formationof the insertion path or insertion of the implant structures 20, so aminimally invasive insertion path sized approximately at or about themaximum outer diameter of the implant structures 20 need be formed.Still, the implant structures 20, which include the elongated bonyin-growth or through-growth regions 24, significantly increase the sizeof the fusion area, from the relatively small surface area of a givenjoint between adjacent bones, to the surface area provided by anelongated bony in-growth or through-growth regions 24. The implantstructures 20 can thereby increase the surface area involved in thefusion and/or stabilization by 3-fold to 4-fold, depending upon thejoint involved.

The implant structures 20 can obviate the need for autologous grafts,bone graft material, additional pedicle screws and/or rods, hollowmodular anchorage screws, cannulated compression screws, cages, orfixation screws. Still, in the physician's discretion, bone graftmaterial and other fixation instrumentation can be used in combinationwith the implant structures 20.

The implant structures 20 make possible surgical techniques that areless invasive than traditional open surgery with no extensive softtissue stripping and no disc removal. The assemblies make possiblestraightforward surgical approaches that complement the minimallyinvasive surgical techniques. The profile and design of the implantstructures 20 minimize rotation and micro-motion. Rigid implantstructures 20 made from titanium provide immediate post-op fusionstability. A bony in-growth region 24 comprising a porous plasma spraycoating with irregular surface supports stable bone fixation/fusion. Theimplant structures 20 and surgical approaches make possible theplacement of larger fusion surface areas designed to maximizepost-surgical weight bearing capacity and provide a biomechanicallyrigorous implant designed specifically to stabilize the heavily loadedlumbar spine.

Although the disclosure hereof is detailed and exact to enable thoseskilled in the art to practice the invention, the physical embodimentsherein disclosed merely exemplify the invention that may be embodied inother specific structure. While the preferred embodiment has beendescribed, the details may be changed without departing from theinvention, which is defined by the claims.

What is claimed is:
 1. An orthopedic implant configured for fusing abone joint, the implant comprising: an elongated bone fixation/fusionimplant body having a central longitudinal axis extending along adirection of elongation of the implant body, and a cross-sectionalprofile transverse to the central longitudinal axis and defined by aplurality of longitudinal apices that extend along the implant bodyparallel to the central longitudinal axis, the implant body having anoverall length in the direction of elongation that is at least 3.6 timesa maximum width of the implant body in a direction transverse to thedirection of elongation, wherein the elongated implant body is providedwith a central lumen extending therethrough and adapted to receive aguide pin to assist in the placement of the implant within the bonesegments, the implant body being sized and configured to be inserted ina direction of the central longitudinal axis, first through a first bonesegment, then transversely across a joint region and then at leastpartially into a second bone segment, the implant being configured to beleft in place in the bone segments postoperatively.
 2. The implant ofclaim 1, wherein the cross-sectional profile of the elongated implantbody is defined by exactly three longitudinal apices.
 3. The implant ofclaim 1, wherein the elongated implant body is provided with a poroussurface configured to be conducive to bony in-growth.
 4. The implant ofclaim 1, wherein the elongated implant body is coated withhydroxyapatite to be conducive to bony in-growth.
 5. The implant ofclaim 1, wherein the implant body has an overall length in the directionof elongation that is no greater than about 6.7 times the maximum widthof the implant body in the direction transverse to the direction ofelongation.
 6. The implant of claim 5, wherein the central lumen has anominal inside diameter that is no more than about 30% of the maximumwidth of the implant body in the direction transverse to the directionof elongation.
 7. An implant system comprising: the implant of claim 6;and a guide pin being configured to be inserted first through the firstbone segment, then transversely across the joint region, and then atleast partially into the second bone segment without preparing a pathfor the guide pin through the bone segments first, the guide pin havinga nominal outside diameter that is the same as the nominal insidediameter of the central lumen such that a close sliding fit is achievedwhen the implant is placed over the guide pin.